This invention relates to improved linear aromatic polyesters of a bisphenol and a dicarboxylic acid. More particularly, the invention relates to such polyesters containing as terminal substituents benzoate ester groups derived from phenyl benzoate.
Linear aromatic polyesters derived from dicarboxylic acids (especially aromatic dicarboxylic acids) and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, discloses linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when processed into useful articles using conventional techniques, e.g. injection molding techniques provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile properties, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation, and good electrical properties.
Although the aforementioned linear aromatic polyesters generally display excellent physical and chemical properties, a persistent and troublesome problem has been their sensitivity to hydrolytic degradation at elevated temperatures. This sensitivity to the combined effects of heat and moisture is also exhibited in commercially available polycarbonate resins as evidenced by the desirability of reducing the water content of the resin to less than about 0.05% prior to molding. Unfortunately, however, the aromatic polyester resins often display a more pronounced tendency to rapidly degrade and embrittle than do polycarbonate resins. This is demonstrated by the loss of tensile strength which can occur when an aromatic polyester resin is molded and subsequently immersed in boiling water. The tendency may be explained, in part, by the hydrolysis of the ester linkages under these conditions. In any event, it is to be appreciated that sensitivity to moisture represents a significant problem in aromatic polyester resins that would significantly limit their commercial utility in applications such as in autoclaves or at elevated temperatures in humid atmospheres.
It has been proposed (according to T. Ueno et al, Japanese Patent No. Sho 53-8696, published Jan. 26, 1978) to enhance the hydrolytic stability of linear aromatic polyesters comprising bisphenol and dicarboxylic monomer residues by carrying out the preparation of the polyester from the bisphenol and dicarboxylic acid diacyl halide reactants in the presence of a monohydric phenolic compound such as p-tert butyl phenol. The monofunctional phenolic compound reacts with terminal carbonyl halide substituents in the polyester to form p-tert, butyl phenyl carboxylate ester end groups. However, modification of the polyester to introduce terminal carboxylate ester groups derived from monofunctional phenols (such as p-tert, butyl phenol) does not enhance the hydrolytic stability to make the products attractive commercially.
The foregoing conventional polyesters are also generally unstable on processing at elevated temperature, e.g. processing by injection molding. This processing instability is evidenced by increased coloration and a significant loss in the polyester intrinsic viscosity and molecular weight during processing.
It is the object of the invention to prepare a structurally modified linear aromatic polyester comprising bisphenol and dicarboxylic acid residues which displays hydrolytic stability.
It is another object of the invention to prepare a structurally modified linear polyester comprising bisphenol and dicarboxylic acid residues of enhanced processing stability.